• Question: What differentiates the harmless and pathogenic bacteria in our gut?

    Asked by Anon to Sabrina on 22 Jun 2020.
    • Photo: Sabrina Slater

      Sabrina Slater answered on 22 Jun 2020:

      Heya 🙂 Great question! Thanks for asking.

      There are a few things that differentiate harmless bacteria and bacterial pathogens. We usually call these harmless bacteria “commensals” or “flora”. In our intestines, commensals help us digest food and provide a protective barrier up against the inside walls of our guts. With this in mind, pathogenic bacteria that cause disease have a few extra things they need to carry around so that they can cause mischief…

      1) They need to hide from by the body, because if they get noticed, they’ll be toast! Some bacteria like Salmonella do this by invading immune cells like macrophages and hiding inside of them. Others like Shigella or Listeria invade normal gut cells and hijack human cell transport systems to jump through from one cell to another. They also have found ways to camouflage the bits of them that are most recognisable, like their flagella (tails they use to swim) or their lipopolysaccharide (their “skin”, if you like) by coating them in different sugars that the body can’t recognise.

      2) They need to be able to fight for nutrients. Often pathogens like Yersinia have what we call siderophores – these are little molecular gadgets that steal metals like iron from the host, which the pathogens need to grow. Some pathogenic bacteria not only feed off of the sugars and small fats released from commensal good guys, but use them as a signal to start making…

      3) … toxins! Certain pathogens can have HUNDREDS of different toxins in a single bacterial cell! Though the range is usually between 2 and 30 different toxins per bacterium. Sometimes these are injected into human cells to mess up all the important reactions they need to do to stay alive. Sometimes they are injected into other bacteria to eliminate competition. Nasty stuff!

      These are the main differences between commensals and pathogens. The cool (or scary?) thing is some pathogens have evolved from regular harmless bacteria by getting hold of bits of DNA that give them some advantage. For example, disease-causing E. coli have managed to expand their genetic code by 120% compared to regular friendly E. coli. This extra 20% is pure weaponry!

      Hope that answers your question! Feel free to ask more about specifics 😀

Comments

  • Photo: anon-256449

    anon-256449 commented on :

    Hi Sabrina, thank you for answering my question (for some reason my name is showing as anonymous!). This is an incredibly interesting topic. How did E. coli actually expand their genetic code? Would humans be able to do this?

    • Photo: Sabrina

      Sabrina commented on :

      Hey Undercover Ella 😛
      Glad you think so too! Bacteria have managed it a few different ways. One way you might have already heard of is horizontal gene transfer, which is a form of DNA exchange that happens between “sister” cells, rather than mother to daughter (offspring cells). To do this the bacterium makes a conjugation pilus, which is kind of like a long hollow tube from its surface, and pokes into sister cells. Genes in the form of DNA fragments are then passed through the pilus and integrated into the recipient bacterium’s genome. This is quite a common way that bacterium become “superbugs”, because they all pass around genes that allow them to be resistant to lots of different groups of antibiotics. Another way that people are paying more and more attention to nowadays is by phage infection. That’s right, bacteria can get infected too! When bacteria get infected by these tiny phages, the phage integrates some of its genes into the bacterial genome (kind of like how viruses do to humans sometimes). The genes that the phages transfer are usually purely for phage survival, but they can accidentally pick up other bits and pieces of DNA when they go around infecting all sorts of different bacteria – so all this stray DNA can get implanted in a bacterium’s genome. This is how a lot of bacterial toxins get passed around!
      Humans can only pass genes in 1 direction – from parent to offspring. BUT, we know that in the past, humans have gotten hold of new genes from viral infection. Nothing like the common colds or flus that go around today mind you, we’re talking ancient history here!